One process for the heavy-metal decontamination of river and harbour sludges, sewage sludges, residues of combustion and pyrolysis, and other relevant problem substances is disclosed in European Patent Specification 0,072,885. The heavy metals are dissolved with mineral acids and then precipitated again as hydroxides with milk of lime at a pH of about 10. The hydroxides are then carbonated by carbon dioxide and the metals still remaining are extracted down to trace amounts. The decontaminated starting substances can then be put to an economically viable use. For example, river and harbour sludges, as well as decontaminated residues of combustion and pyrolysis, may be used for the production of bricks, light-weight building materials or cement building materials. Decontaminated sewage sludges can be used as fertilizers. In each of these cases, problem-free disposal is possible.
The heavy-metal hydroxides and carbonates obtained in the decontamination are themselves valuable raw materials. It would therefore be desirable to separate these valuable raw materials. However, since these heavy metals typically have a high iron content, e.g. 90% of the total metal content, the use of these raw materials is inhibited. Recycling the valuable nonferrous heavy metals is therefore of little interest, particularly at times when there is an inexpensive surplus of metal ores and scrap metal on the world market.
One process disclosed in the above-referenced European Patent Specification teaches to first precipitate the bulk of the iron together with some of the heavy metals at pH 7 and then to precipitate the remaining heavy metals at pH 10. The iron fraction obtained according to this process and the heavy metal fraction precipitated at a higher pH value, however, both consist predominantly of iron and, therefore, cannot be readily used as raw materials in the metallurgical industry.
The heavy-metal hydroxide/carbonate mixtures precipitated using known decontamination processes must, therefore, be largely disposed of as expensive and undesirable "special waste", i.e. as waste material which cannot be eliminated safely with normal domestic and commercial refuse.
While various processes are known for the separation of heavy metals, e.g. sulphide separation process, separation by complexing, and separation by ion exchange, such separation processes for heavy metals have significant drawbacks. Either the costs of the chemicals or equipment are too high, or the separation processes are not sufficiently specific. The sulphide separation process would certainly make good separation possible but, due to environmental reasons, it is almost impossible to implement today and entails high investment costs for the necessary safety measures.
In practice, therefore, there is a need for simple, cost-effective and technologically viable ways of separating the iron, if possible quantitatively, from the remaining heavy metals and also of separating the heavy metals from each other individually or at least in groups so that they become economically viable raw materials for the metallurgical industry.